1. Field of the Invention
This invention relates generally to methods and apparatus for imaging objects in a liquid environment and, more particularly, to imaging objects with acoustic waves.
2. State of the Art
Various types of acoustic imaging processes have been developed over the years in which an acoustic wave is used to collect information relating to certain features and structures of objects. Acoustic imaging processes are useful for applications where an opaque or semiopaque liquid such as oil or polluted water or solids, impair optical imaging approaches and techniques. Acoustic waves easily travel through such media and can provide images of objects in such liquid media.
Conventional acoustic imaging approaches have typically taken two approaches. In one approach, ultrasonic imaging techniques utilize an array of “microphones” to detect acoustic amplitude and phase changes and create an image based upon a reflected acoustic wave. With such an approach, acoustic images are formed from an analysis of amplitude and phase relationships between reflected acoustic signals at various points in the array. Such a direct acoustic detection system is limited in resolution based upon the practical fabrication limitations of the sensors and interaction of adjacent sensors. Even when piezoelectric pressure sensors are utilized, coarse resolutions of 100-by-100 pixels have become cumbersome due to signal routing and crosstalk shortcomings. Such a discrete or pixelized approach is limited in resolution, in large measure, to the coarse granularity of available sensor arrays. Furthermore, arrays fabricated from discrete transducer elements have limitations brought about by the number of elements and the complexity of the electronic readout of the information from the array.
A second conventional ultrasonic approach includes scanning or rasterizing a single “microphone” across a target object to detect the acoustic reflections at various times and spatial locations. Such an approach may employ a single acoustic pulse detected across a broad spatial region or may utilize successive acoustic pulses across the spatial region. Such a scanning approach requires significant control parameters to determine time delays from each sample and further requires consistent scanning control. Additionally, sophisticated signal processing of time delays and phasing of each sample imposes further resolution limitations on a system.
Both conventional approaches present pixelization of the acoustic image at an early stage and by rudimentary acoustic sensors. Consequently, a need exists for a method and apparatus for more readily imaging objects in a liquid environment that do not suffer from the shortcomings of the prior art approaches.